This invention relates generally to electronic radio frequency identification tags and specifically to small electronic transponders that store and transmit information. In particular, the present invention relates to the use of light to generate sufficient power for transponders.
Electronic transponders are used in a wide variety of applications to store and transmit information. A transponder functions by receiving a transmission request and, in turn, transmitting a response. Typically, this response is an identification signal, often comprising a serial number.
In World War II, transponders were used to identify aircraft. The transponder assured the requesting aircraft that the associated aircraft was a friendly aircraft by transmitting an identification code. Early versions of electronic transponders supplied power by way of a battery or a solenoid. However, batteries and solenoids are relatively large, and therefore severely restrict the ability to reduce the size of electronic transponders.
An antenna external to the transponder broadcasted identification information. This external antenna was necessary to generate a RF signal strong enough to be received and demodulated by a receiver. An external antenna, however, further increases the size of the transponder.
Today, transponders are used for a variety of purposes ranging from identification of wildlife to electronic article surveillance (EAS). Typically, transponders utilize a radio frequency identification (RFID) system. These systems operate without visual contact. For example, EAS systems typically employ a closed loop of a conductive substance that responds to a generated radio frequency (RF) field. These transponders, also called tags due to their ability to “tag” a consumer item to prevent shoplifting, are deactivated when a product is purchased. To further this goal, EAS systems may transmit a description of the item to which the tag is affixed.
Transponders are also beneficial for applications where it is highly desirable to reduce the size of the transponder to very small dimensions. For example, electronic transponders aid in the detection of biomolecules in samples when performing solid-phase assays. U.S. Pat. Nos. 5,641,634, 5,736,332, 5,981,166, and 6,001,571 respectively disclose the use of transponders for detecting biomolecules, determining the sequence of nucleic acids, screening chemical compounds, and performing multiplex assays for nucleic acids, and are herein specifically incorporated by reference. For these applications, the transponder must be significantly reduced in size.
For use in chemically hostile environments, as those often used in solid-phase assays, external antennas and power sources utilized in earlier prior art transponders needed to be protected. Therefore, the entire transponder, including the power source and antenna, would be enclosed in a protective material, such as a glass bead. This enclosure further added to the size of the transponder.
As disclosed in U.S. Pat. No. 5,641,634, miniature transponders, also referred to as microtransponders, using photovoltaic cells to provide power have been developed. Photo-activated transponders enable smaller dimensions. Further, by providing a monolithic assembly that includes an antenna, the transponder disclosed in U.S. Pat. No. 5,641,634 further enables a reduction in size.
These transponders are typically formed on a silicon wafer and protected by a thin layer of silicon dioxide (SiO2). SiO2 has the same chemical properties as glass with respect to chemically hostile environments. Therefore, the transponder does not need to be enclosed in a glass encasement. Alternatively, the transponder may be coated with a variety of transparent or semitransparent materials, such as plastic or latex.
In many applications, it is desirable to have a small transponder that outputs identification data. It is further desirable to create a purely passive device that does not depend on the operation of self-contained batteries. Photo-activated transponders provided an advantage over the prior art due to their inactivity without light illumination. A narrowly focused laser light source may enable a single transponder at a time, even when a large number of transponders are present in the assay. Only the illuminated transponder transmits data and other transponders are inactive. The reduction in the number of transmitting transponders significantly reduces noise level. If the light source is more broadly applied, an increased number of transponders may respond. Thus, the light source can be adjusted to control which transponders will respond during an assay.